Electrophysiological Investigation of the Projection of Secondary Muscle Spindle Afferents in the Cat Spinal Cord

The intraspinal projection of functionally identified secondary muscle spindle afferents from tenuissimus and medial gastrocnemius was investigated in anesthetized cats. Centrally and peripherally intact single fibres were recorded in the L7 dorsal root. The spinal projection of their axonal branches was traced out by antidromic intraspinal microstimulation (lowest threshold 1.6‐6.0 μA). Collaterals of the investigated fibres were found not only in the dorsal funiculus, the dorsal horn and the intermediate region, but also in the ventral horn close to the motor nucleus or even reaching into it. These findings in conjunction with the results regarding group II focal synaptic potentials (Fu et al. 1974) suggest the existence of two main populations of first order interneurones activated from secondary spindle afferents located in the dorsal and ventral horn, respectively. The central delay of the antidromically conducted group II impulses indicated a substantial reduction of their effective conduction velocity within the spinal cord.